Gasification is a high temperature/pressure chemical process used to convert a carbon feedstock into CO and H2 for use in power generation and the production of chemicals. It is also a leading candidate as a source of hydrogen in a hydrogen economy. Gasifiers, where the gasification reaction occurs, are processing vessels that contain the high temperature/pressure chemical reactions between a carbon feedstock (typically coal and/or petcoke), water, and oxygen (reducing); producing CO and H2 gas (also called synthesis gas, or syngas). The gasification reaction occurs according to the following simplified reaction: C+H2O+O2→H2+CO+CO2+H2S+minority gases+by products. Gasifiers are also being considered a critical component of advanced power generation technologies targeting near zero emissions; in part because gasification generates very low emissions, and because CO2 generated by the gasification process can be easily captured for sequestration/reuse
Several types of gasifiers are in commercial production, including dry ash and slagging variations. Slagging gasifiers generally operate at temperatures between 1250°−1575° C., at pressures between 300-1000 psi, and at oxygen partial pressures around 10−4 to 10−10 atm. Newly constructed gasification facilities may target production of up to 38,000 mtd of carbon feedstock, which is typically either coal and petroleum coke (also called petcoke), or combinations of them. Most impurities in coal and petcoke originate from mineral impurities, and range from about 1 wt % for petcoke to 10 wt % or higher for coal. The major impurities of concern in coal feedstock are oxides of Si, Fe, Ca, and Al; with additional oxide impurities of Ni and V in petcoke. During gasification, the impurities in the carbon feedstock liquefy, forming a slag which attacks the refractory liner currently used to protect the high pressure containment shell where gasification occurs, resulting in significant wear. Interactions between the slag and the refractory liner occur through the two major wear mechanisms of: 1) chemical dissolution and 2) spalling.
Structural spalling is caused by slag penetration into the porous refractory as it flows down the gasifier sidewall, and differences in physical properties between the penetrated and un-penetrated layers. Coal carbon feedstock rates of 2400 tons/day to a slagging gasifier create about 240 tons per day of slag waste, with the majority of the slag created flowing over the surface of a refractory. The mitigation of slag penetration into the refractory of air-cooled slagging gasifiers is a source of significant effort in order to reduce refractory wear and increase service life.
High chrome oxide materials have evolved as the best refractory linings in air cooled slagging gasifiers. The interaction between FeO in the slag and Cr2O3 in the refractory is highly beneficial in that it leads to a chemistry change in the slag, which increases the slag viscosity and limits its penetration into the refractory microstructure. See e.g. Chan et al., “Effect of Cr2O3 on Slag Resistance of Al2O3—SiO2 Refractories”, J. Am. Ceram. Soc. 75 (1992), among others. Identified high chrome oxide refractories include compositions comprised of Cr2O3—Al2O3, Cr2O3—Al2O3—ZrO2, and Cr2O3—MgO; with the use of Cr2O3—MgO being limited or discontinued in the United States because of concerns over hexivalent chrome oxide. Generally, the refractories exhibit a microstructure having a high porosity (typically 10-20%). Porosity is an artifact of the controlled grain sizing used in making the refractory and of the sintering process, and helps to control thermal shock resistance. However, the porosity also allows slag to infiltrate the grains and grain boundaries, leading to slag/refractory interactions/corrosion, resulting in one of the primary failure modes of structural spalling.
It would be advantageous to provide a refractory brick for gasifier environments whereby a material could be used to fill refractory pores and mitigate the penetration of slag into the refractory. Such an improvements in the refractory microstructure would have a significant influence on wear caused by structural spalling. It would be additionally advantageous if the material were compatible with the high chromium content of refractory brick used in typical gasifiers, so that interactions between the material and the chromium leading to reductions in the chromium inventory could be avoided.
A technique to reduce wear that that has proven successful is the inclusion of carbon in the porous structure of a pre-fired refractory. Magnesia-carbon bricks are well-established refractory products in the steel industry. Similarly, refractory compositions of alumina, silicon, carbon, and chromic oxide are known. See U.S. Pat. No. 4,210,454 to Rechter, issued Jul. 1, 1980. The alumina-carbon-chromic oxide refractory includes silicon to mitigate the oxidation of the carbon, and the compositions generally limit chromic oxide to about 15 wt % of the composition. For air cooled slagging gasifier applications, this limit on chromic oxide composition is significantly less than that desired for economic feasibility. Studies conducted to determine the stability of Al2O3-rich refractories show that the higher the Cr2O3 concentration, the lower the deterioration rate. See Rawers et al., “Characterizing coal-gasifier slag-refractory interactions”, Materials at High Temperatures, 16 (4) (1999). Generally, for air-cooled slagging gasifier environments, 60-95 wt % chromium oxide is utilized in Cr2O3—Al2O3 refractories. However, the inclusion of carbon in pre-fired chromia refractories intended for low oxygen partial pressure environments (10−4-10−10 PO2) has generally been avoided because, under the applicable temperature and oxygen partial pressure conditions, chromia (Cr2O3) and carbon are thermodynamically predicted to form chromium carbides. Formation of these chromium carbides would act to both reduce the overall chromia inventory as well as negatively impact the ceramic bonding between grains that exists in such a pre-fired structure, leading to significant compromise of the pre-fired brick's structural integrity and a significantly negative impact on the operating life of gasifer refractory linings constructed with such pre-fired chromia bricks. For this reason, carbon inclusion within the pores of pre-fired chromia bricks has been avoided.
There have been efforts to incorporate chromia and carbon in pressed, resin-based, carbon-bonded bricks. See e.g., U.S. Pat. No. 8,658,552 issued to Prior et al., issued Feb. 25, 2014. The '552 patent provides pressed bricks having compositions generally comprising chrome aluminum aggregate, chromic oxide, aluminum powder, 1-8 wt. % of a phenolic resin, and typically about 2 wt. % carbon black. The resulting refractory is a carbon-bonded refractory, and additional metals are typically added to the composition to serve as anti-oxidants in service and preserve the bonding. In testing, chromia reduction is noted and is generally attributed to the presence of the resin bond and the carbon black. The '552 patent acknowledges this loss of chromia but accepts it as a trade-off due to the reduced slag penetration noted.
It would be advantageous to provide a chromia refractory brick for gasifier operations able to utilize carbon for mitigation of slag penetration and compatible with the high chromium oxide requirements generally established for applications in the low oxygen partial pressure environment of the gasifier, where the carbon and chromia were established in a relationship that avoided significant chromia inventory reductions in service. It would be further advantageous to provide a refractory brick which is not reliant on additional components such as silicon for mitigation of carbon oxidation, or other additives that would negatively impact the refractory service life performance.
Provided herein is a chromia refractory brick comprising a porous refractory brick comprising at least 60 wt. % Cr2O3, the balance generally being composed primarily of up to 40 wt pct Al2O3 or other refractory oxides. The porous refractory has a grain structure where the grains are bound to one another grain by ceramic rather than carbon bonding, where the ceramic bond is a solid state solution having a composition MaOb and a substantial absence of carbon. Typically, M comprises Cr, Al, or a combination. The ceramically-bonded porous refractory additionally has an interconnected porosity greater than about 9% and generally less than about 20%. Within this ceramically-bonded, porous structure, carbon deposits reside within the plurality of pores and is present from about 1 wt. % to less than about 10 wt. % of the chromia refractory brick. The porous refractory may be formed by pre-firing a mixture of aggregate and fine powders such as Cr2O3, Al2O3, FeO, TiO2, MgO, or other oxides at temperatures generally exceeding 1300° C. to a glassy or solid state oxide bond between larger oxide particles through solid state diffusion, liquid coalescence, vapor transport, or other known transport material transport means. Following formation of the ceramically-bonded, porous refractory, the pores remaining pores are filled with a carbon generating material, such as pitch and tar, followed by coking. The disclosure thus provides a high chromia refractory brick for gasifier environments where slag penetration is mitigated through use of carbon arranged in a manner compatible with the high chromium content of the refractory brick, so that reductions in the chromium inventory or a significant decreases in material performance can be avoided.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.